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The quantum mind or quantum consciousness hypothesis proposes that classical mechanics cannot explain consciousness, while quantum mechanical phenomena, such as quantum entanglement and superposition, may play an important part in the brain's function, and could form the basis of an explanation of consciousness. It is not one theory, but a collection of distinct ideas described below.
A few theoretical physicists have argued that classical physics is intrinsically incapable of explaining the holistic aspects of consciousness, whereas quantum mechanics can. The idea that quantum theory has something to do with the workings of the mind go back to Eugene Wigner, who assumed that the wave function collapses due to its interaction with consciousness. However, modern physicists and philosophers consider the arguments for an important role of quantum phenomena to be unconvincing.^{[1]} Physicist Victor Stenger characterized quantum consciousness as a "myth" having "no scientific basis" that "should take its place along with gods, unicorns and dragons."^{[2]}
The philosopher David Chalmers has argued against quantum consciousness. He has discussed how quantum mechanics may relate to dualistic consciousness.^{[3]} Indeed, Chalmers is skeptical of the ability of any new physics to resolve the hard problem of consciousness.^{[4]}^{[5]}
David Bohm took the view that quantum theory and relativity contradicted one another, and that this contradiction implied that there existed a more fundamental level in the physical universe.^{[6]} He claimed that both quantum theory and relativity pointed towards this deeper theory, which he formulated in terms of a quantum field theory. This more fundamental level was proposed to represent an undivided wholeness and an implicate order, from which arises the explicate order of the universe as we experience it.
Bohm's proposed implicate order applies both to matter and consciousness, and he suggests that it could explain the relationship between them. Mind and matter are here seen as projections into our explicate order from the underlying reality of the implicate order. Bohm claims that when we look at the matter in space, we can see nothing in these concepts that helps us to understand consciousness.
In trying to describe the nature of consciousness, Bohm discusses the experience of listening to music. He thinks that the feeling of movement and change that make up our experience of music derives from both the immediate past and the present both being held in the brain together, with the notes from the past seen as transformations rather than memories. The notes that were implicate in the immediate past are seen as becoming explicate in the present. Bohm views this as consciousness emerging from the implicate order.
Bohm sees the movement, change or flow and also the coherence of experiences, such as listening to music as a manifestation of the implicate order. He claims to derive evidence for this from the work of Jean Piaget^{[7]} in studying infants. He states that these studies show that young children have to learn about time and space, because they are part of the explicate order, but have a "hardwired" understanding of movement, because it is part of the implicate order. He compares this "hardwiring" to Chomsky's theory that grammar is "hardwired" into young human brains.
In his writings, Bohm never proposed any specific brain mechanism by which his implicate order could emerge in a way that was relevant to consciousness, nor any means by which the propositions could be tested or falsified.
This section relies on references to primary sources. (February 2012) 
Theoretical physicist Roger Penrose and anaesthesiologist Stuart Hameroff collaborated to produce the theory known as Orchestrated Objective Reduction (OrchOR). Penrose and Hameroff initially developed their ideas separately, and only later collaborated to produce OrchOR in the early 1990s. The theory was reviewed and updated by the original authors in late 2013.^{[8]}
Penrose's controversial argument began from Gödel's incompleteness theorems. In his first book on consciousness, The Emperor's New Mind (1989), he argued that while a formal proof system cannot prove its own inconsistency, Gödelunprovable results are provable by human mathematicians. He took this disparity to mean that human mathematicians are not describable as formal proof systems, and are not therefore running a computable algorithm.
Penrose determined that wave function collapse was the only possible physical basis for a noncomputable process. Dissatisfied with its randomness, Penrose proposed a new form of wave function collapse that occurred in isolation, called objective reduction. He suggested that each quantum superposition has its own piece of spacetime curvature, and when these become separated by more than one Planck length, they become unstable and collapse. Penrose suggested that objective reduction represented neither randomness nor algorithmic processing, but instead a noncomputable influence in spacetime geometry from which mathematical understanding and, by later extension, consciousness derived.
Originally, Penrose lacked a detailed proposal for how quantum processing could be implemented in the brain. However, Hameroff read Penrose's work, and suggested that microtubules would be suitable candidates.
Microtubules are composed of tubulin protein dimer subunits. The tubulin dimers each have hydrophobic pockets that are 8 nm apart, and which may contain delocalised pi electrons. Tubulins have other smaller nonpolar regions that contain pi electronrich indole rings separated by only about 2 nm. Hameroff proposes that these electrons are close enough to become quantum entangled.^{[9]} Hameroff originally suggested the tubulinsubunit electrons would form a Bose–Einstein condensate, but this was discredited.^{[10]} He then proposed a Frohlich condensate, a hypothetical coherent oscillation of dipolar molecules. However, this too has been experimentally discredited.^{[11]}
Furthermore, he proposed that condensates in one neuron could extend to many others via gap junctions between neurons, thus forming a macroscopic quantum feature across an extended area of the brain. When the wave function of this extended condensate collapsed, it was suggested to noncomputationally access mathematical understanding and ultimately conscious experience, that are hypothetically embedded in the geometry of spacetime.
However, OrchOR made numerous false biological predictions, and is considered to be an extremely poor model of brain physiology. The proposed predominance of 'A' lattice microtubules, more suitable for information processing, was falsified by Kikkawa et al.,^{[12]}^{[13]} who showed that all in vivo microtubules have a 'B' lattice and a seam. The proposed existence of gap junctions between neurons and glial cells was also falsified.^{[14]} OrchOR predicted that microtubule coherence reaches the synapses via dendritic lamellar bodies (DLBs), however De Zeeuw et al. proved this impossible,^{[15]} by showing that DLBs are located micrometers away from gap junctions.^{[16]}
Hiroomi Umezawa and collaborators proposed a quantum field theory of memory storage. Giuseppe Vitiello and Walter Freeman have proposed a dialog model of the mind, where this dialog takes place between the classical and the quantum parts of the brain.^{[17]}^{[18]} Quantum field theory models of brain dynamics are fundamentally different from the PenroseHameroff theory. Subhash Kak has proposed that the physical substratum to neural networks has a quantum basis,^{[19]} but he also points out that the quantum mind will still have machinelike limitations.^{[20]}
Henry Stapp favors the idea that quantum waves are reduced only when they interact with consciousness. He argues from the Orthodox Quantum Mechanics of John von Neumann that the quantum state collapses when the observer selects one among the alternative quantum possibilities as a basis for future action. The collapse, therefore, takes place in the expectation that the observer associated with the state.
His theory of how mind may interact with matter via quantum processes in the brain differs from that of Penrose and Hameroff.^{[21]}
The main argument against the quantum mind proposition is that quantum states in the brain would decohere before they reached a spatial or temporal scale at which they could be useful for neural processing. This argument was elaborated by the physicist, Max Tegmark. Based on his calculations, Tegmark concluded that quantum systems in the brain decohere quickly and cannot control brain function.^{[22]}^{[23]}